The principal investigator has produced a very valuable reagent, Fas-Ig, which will allow him to assess the role of the Fas/FasL interaction in central and peripheral tolerance. The specific aims are to: 1) Determine whether Fas/FasL interactions participate in the deletion of antigen- specific CD4+8+ thymocytes in vitro. 2) Demonstrate a role for the Fas/FasL interaction in clonal deletion in vivo by injecting Fas-Ig. 3) Demonstrate a role for the Fas/FasL interaction in clonal deletion in vivo by chronically administering Fas-Ig. 4. Study the molecular pathway of Fas/FasL co-expression following TCR-dependent activation. 5. Study the molecular regulation of the FasL gene.